Applicants claim priority of Japanese patent application, Ser. No. 2000-093638, filed Mar. 30, 2000.
This invention relates to a carburetor fuel mixture adjustment assembly, and more particularly to a carburetor fuel mixture adjustment needle valve assembly for an internal combustion engine.
It is known for a conventional fuel mixture adjustment assembly 70 as shown in FIG. 6, to include a cylindrical needle valve body 50 disposed rotateably within an elongated cylindrical needle valve receptacle 46 in a carburetor body 61 to adjust fuel flow. The fuel flows through a fuel passage 42 from a fuel metering chamber, not shown, and into a fuel-and-air mixing passage 41 via a fuel jet. The fuel passage 42 is intersected by the receptacle 46 at an inner distal end or fuel chamber 44. Rotation of the needle valve body 50 causes a stem portion 50 having a needle tip 51 of the needle valve body 50 to axially advance into, or retract out of, a fuel chamber 44 of the receptacle 46. Fuel flows traversely into chamber 44 from an inlet orifice, not shown, which communicates through the cylindrical wall of chamber 44, and flows out of an outlet passage or orifice 43 aligned concentrically to and communicating axially inward of the fuel chamber 44. The needle tip 51 projects concentrically into the outlet orifice 43 thereby obstructing fuel flow. The stem portion 52 is spaced or separated radially inward from the wall of the fuel chamber 44 thereby permitting fuel flow between the inlet and outlet orifices.
Axial advancement and retraction of the needle tip 51 within the outlet orifice 43 respectively decreases and increases the amount of fuel that can flow through the orifice 43 by decreasing and increasing the cross-sectional area of the valve restriction through at the orifice 43. An exteriorly threading portion 57 of the needle valve body 50 disposed concentrically to and axially outward from the stem portion 52 is directly threaded to a cylindrical wall 58 of the carburetor body 61 exposed within the receptacle 46. The needle valve body 50 is rotated by using a tool such as a screwdriver to engage a screw head 58 of the valve body 50 that protrudes from the carburetor body 61. In some such assemblies 70, to prevent inadvertent or uncommanded rotation of the needle valve body 50 within the needle valve receptacle 46, a tamper-resistant adjustment needle limiter cap is placed over the screw head 50 and is secured to or braced against an adjacent structure, not shown.
Fuel mixture adjustment assemblies 70 of this type include enough clearance between the respective threading portions 57 of the needle valve body 50 and the needle valve receptacle 46 to allow for lateral movement of the needle tip 51 within the outlet orifice 43 when force is applied to the head 58 of the needle valve body 50. This lateral movement can change the size of the orifice 43 enough to result in fuel flow rate changes of up to twenty percent from an optimum fuel flow rate determined by the manufacturer. Fuel flow rate changes caused by needle xe2x80x9cslopxe2x80x9d0 result in excessively rich or lean fuel mixtures that undesirably increase exhaust emissions. Therefore, it is desirable to reduce fuel flow fluctuations through the needle valve assembly and the resulting increase in exhaust emissions by limiting needle slop.
Suppressing unintentional rotation of the needle valve body 50 (possibly from engine vibration) which would result in inadvertent alteration of the fuel mixture is a spring 56 disposed concentrically about the needle valve body 50 and compressed axially. A conventional O-ring 54 prevents the leakage of air through the loose fitted threads within the receptacle 46 and into the sub-atmospheric pressure fuel chamber 46 thereby preventing unstable engine operation. The O-ring 54 seals against a conical annular surface 45 of the carburetor body 61 and a shank portion 53 engaged concentrically between the stem portion 52 and the threading portion 57 of the needle valve body 50. Since the diameter of the shank portion 53 is less than the diameter of the threading portion 57 the shank portion 53 has an axial inward facing annular surface. The spring 56 is compressed axially between the O-ring 54 and the shank portion 53 or annular surface. The axial forces produced by the compressed spring 56 suppress rotation of the needle valve body 50 and assures that the O-ring 54 remains seated sealably between the carburetor body 61 and the shank portion 53 of the needle valve body 50. Unfortunately, the O-ring and spring are two additional parts which are costly to manufacture and assemble in the carburetor.
A carburetor fuel mixture adjustment assembly has a threadable holder which suppresses uncommanded rotation of a needle valve body disposed threadably through the threadable holder within an elongated receptacle extending into a carburetor body. A needle portion of the needle valve body advances and retracts into and out of a fuel passage, which feeds fuel to a fuel-and-air mixing passage within the carburetor, via rotation of the needle valve body to respectively decrease and increase fuel flow through the fuel passage. Preferably, a sealing holder disposed within the receptacle has a stem holder portion and a shank holder portion. The stem holder portion holds a stem portion and thereby assures that a needle portion of the needle valve body remains concentrically disposed within an outlet orifice of a fuel chamber of the receptacle which intersects the fuel passage. The shank holder portion disposed within a sealing chamber of the receptacle provides a seal radially between the needle valve body and a sealing chamber wall of the carburetor body preventing air ingress to the sub-atmospheric fuel chamber.
The threadable holder is prevented from rotating within the receptacle via engagement of a clockwise rotational stop surface and a counterclockwise rotational stop surface of the carburetor body with respective clockwise facing surface and counterclockwise facing surface of the threadable holder. The threadable holder prevents rotation of the sealing holder within the receptacle by engagement of a first rotational stop of the shank holder portion with the clockwise facing surface of the threadable holder and engagment of a second rotational stop of the shank holder portion with the counterclockwise facing surface of the threadable holder. The threadable holder is made of a softer material than the needle valve body so that the threading portion of the needle valve body is capable of tapping or forming thread grooves into the rigidly held threadable holder as the needle valve body is screwed into the receptacle.
Preferably, the threadable holder is tightly fitted within a threading chamber of the receptacle. The bottom of the threading chamber is defined by an axial inner shelf and an axial outer shelf which both face outward with respect to the carburetor body. The ends of the inner and outer shelves are interconnected about a centerline of the receptacle by the clockwise and counter clockwise rotational stop surfaces of the carburetor body. Extended axially inward from the inner perimeter of the resultant annular shelf of the threading chamber is a sealing chamber wherein the shank holder portion of the sealing holder is tightly fitted. Inward of the sealing chamber is a fuel chamber. The shank portion of the needle valve body disposed between the needle portion and the threading portion is axially aligned to the sealing chamber and the needle portion is substantially axially aligned to the fuel chamber.
Objects, features and advantages of this invention include the prevention of air ingress into the fuel passage through the receptacle when the fuel chamber is self atmospheric and the prevention of fuel leakage through the receptacle when the fuel chamber is at atmospheric pressure. Furthermore, the needle portion is stabilized or centered concentrically within the fuel chamber. Yet another advantage, is the sealable thread feature between the threadable holder and the threading portion of the needle valve body preventing looseness of the body and uncommanded rotation of the needle valve body which can alter fuel mixture increasing exhaust emissions. The cost of manufacturing is reduced by the elimination of the O-ring and spring utilized for a conventional carburetor fuel mixture adjustment assembly.